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Active Filters

Filters often contain embedded amplifiers between passive-filter stages as shown in figure 6.13.

Figure 6.13:  Buffer amplifier as part of active filter.

These filters have a limited performance since the poles are still real and hence the knees are not sharp. For example, a three stage high-pass filter with buffer amplifiers has a transfer function

and only drops 18 db/octave.

For complex poles we must use either integrators or differentiators. Consider figure 6.14.

Figure 6.14:  Active filter with complex poles.

The closed-loop gain is

By exchanging the input resistor for a capacitor we can change between a low-pass and high-pass filter.

Doug Gingrich
Tue Jul 13 16:55:15 EDT 1999